Many neurological pathologies occur when cells react either normally or aberrantly to extracellular stress stimuli. As such, knowledge of the signaling pathways responsible for relaying extracellular stress cues to the cell's core regulatory machinery is of particular importance in the understanding of disease etiology. One protein known to play a key role in the transduction of cellular stress stimuli is the serine/threonine protein kinase Apoptosis Signal-regulating Kinase 1 (ASK1). Indeed, ASK1 activity has been shown to play a critical role in the endoplasmic reticulum stress-induced cytoxcity resulting from beta-amyloid and huntingtin aggregates in Alzheimer's and Huntington's Diseases, respectively. Owing to its critical role in neuronal cell fate, the activity of ASK1 is tightly regulated;however, the molecular mechanisms governing this regulation remain incompletely understood. Preliminary findings indicate that ASK1 participates in a growth factor- dependent interaction with the recently identified Proline-Rich Akt Substrate of 40kDa (PRAS40). Interestingly, in addition to its reported role in suppressing mTOR function, PRAS40 has also been found to play a protective role in in vivo models of neuronal cell death;however, the mechanism of this pro-survival function remains unkown. In this proposal, the hypothesis that PRAS40 modulates ASK1 activity and function via direct protein- protein interaction will be tested. The following specific aims are proposed to address this hypothesis: 1.) To characterize the structural and physiological determinants of PRAS40/ASK1 binding;2.) To determine whether PRAS40 modulates ASK1 activity;and 3.) To determine whether PRAS40 modulates ASK1-dependent physiological or pathological effects. The long-term goal of this project is to gain a more detailed understanding of the mechanisms by which critical intracellular signaling pathways cooperate in order to regulate cellular stress responses. Such an understanding will encourage the rational development of novel, more finely tuned therapeutic interventions for a wide range of diseases involving stress-induced neuronal cell death, including Huntington's and Alzheimer's Diseases, amyotrophic lateral sclerosis, and traumatic spinal cord and brain injuries.